There are a wide number of applications in industry and elsewhere where fluid is controlled within a gas or hydraulic system of some type. These fluid handling systems often involve valves which may regulate flow, not only from an on/off standpoint, but also provide intermittent flow modulation. In particular, flow control devices are known that are used in high pressure applications and typically include a valve trim which is a sort of flow restrictor. Valve trims can provide several advantages, particularly in the case of high pressures.
Typical valve trims are comprised of a single orifice, where the pressure is reduced in a single stage of pressure drop. Although the invention has applications with both compressible and non-compressible fluid flow, the major source of valve noise is aerodynamic noise in compressible fluid systems. Aerodynamic noise is noise generated from having Reynolds stresses or shear forces in a turbulent flow stream resulting from deceleration, expansion, or impingement. The principal area of noise generation in a control valve is the recovery region immediately downstream of the vena contracta, where the flow field is characterized by intense mixing and turbulence.
The best way to reduce valve generated noise is to reduce noise at the source, which is at the valve trim. Disc stack technology has provided several advantages over standard single stage valve trim with regard to noise reduction in a fluid handling system. One advantage that disc stack technology provides is the design uses multi-path flow geometry, where the flow stream is subdivided into many small paths. The fluid energy at the outlet of many small flow paths is much lower than at the outlet of a single large flow path of equivalent area. Multi-path trim designs are known to provide noise levels that are up to 15 dBA lower than standard trim. Another advantage is that the flow paths are configured to have a multistage pressure drop. This reduces the turbulence and energy release at each stage, reducing the overall generated noise. Disc stack valves also control fluid velocity, which is a noise generator in all fluid systems, and typically have an expanding flow path to reduce velocity and allow for fluid expansion.
The characteristic of disc stack trims for having a gradual pressure decrease is beneficial in permitting a valve to move between open, closed and intermediate flow positions without subjecting the entire system to excessive shocks. Another benefit of known valve trims is that they can provide noise attenuation within the fluid handling system. By gradually reducing the flow pressure in multiple stages over the valve trim area, the valve trims have proven very effective in reducing noise.
The geometry of the fluid path that is formed can take on a variety of configurations. The individual discs are assembled to create the so-called “disc stack” trims where a fluid restrictor is provided in connection with a valve. In one type of arrangement, a disc stack having a number of convoluted radial fluid paths is provided with a control element in the form of a fluid restriction or a plug centrally moveably provided within the disc stack. The fluid restrictor or plug is moved within the disc stack to expose a greater or smaller number of fluid paths thus controlling the amount of fluid flow. In addition to creating less valve-generated noise, such disc stacks are capable of providing a benefit of silencing existing noise in a flow stream as well. Besides the use of disc stacks in connection with the valve itself, the stacks have also been used as a silencer placed down stream of a control valve or at the end of a pipeline where it is desirable to reduce fluid pressure in a quiet manner.
There are many known arrangements of disc stack technology in industry. In general, in these devices, the convoluted flow path is formed as a series of radial grooves in a single disc that are torturous in nature. A number of multi-path, multistage discs are stacked on top of one another to form a cylindrical shaped disc stack. As mentioned above, the paths in the disc can be blocked or exposed by a plug moveably disposed in the center of the layer of discs. Because the paths are torturous, in that they provide a number of obstacles, right angle turns, expansion in the flow path, and a relative long overall flow path which creates frictional resistance, the pressure in the fluid as it travels through the path is reduced in a way that controls the velocity of the fluid. Because high velocity fluid can be a source of noise generation, reduction in velocity reduces noise, and the fluid also exits the valve trim at a much lower velocity compared to if a single orifice were used, thus allowing the valve trim to provide quiet operation compared to a single orifice.
While the above described noise reducing systems have proven to be very successful, it would be desirable to have even lower noise generation performance than is provided by the known disc stacks. Accordingly, there is a need in the art for sound attenuation devices and methods that can provide enhanced performance and/or convenience of manufacture and use in some applications.